Generate a oneshot everytime the pulse is greater then 3sec.
The oneshot will Start a timer TOF with a preset of 2min.
Create four memorys to store time in.
When a oneshot is triggered, save the elapsed time of the TOF to memory1. And then reset the timer. This will calculate the time between pulses.
The next time the oneshot is triggered repeat the above step and save it to memory2.
The next time the oneshot is triggered repeat the above step and save it to memory3.
The next time the oneshot is triggered repeat the above step and save it to memory4.
Just repeat this and if all memory1-4 are full, begin at memory1 again.
Add the memory1-4 and compare it to two minutes. If it's under, set the alarm.
Don't know if it's going to work but that's what I had in mind